[Photo provided to China Daily]

"The submersible relies on throwing out 'ballast pigs' (heavy iron cubes) to gain buoyancy, but one of the cubes was stuck on Jiaolong's waist. It means it could not plunge or rise, but halted at a depth of 1,000 meters," Liao says.

Multiple emergency systems are designed for the vessel, including unloading the battery or robot-mounted arms, but all failed. Relying on the last hope - using the propeller, despite a power shortage after a full day of research under the sea - the sub pilot returned the craft to the surface.

"The young man was scared, but he didn't tell the passengers. The next day when we interviewed him, he said he once prayed for survival, as he is the only child in his family," recalls Liao.

For Liao and his crew, the Deep Sea series has not been a documentary seeking eyeball-grabbing adventures, but an examination of the relationship between nature and human beings.

"Our distant ancestors lived in the ocean around 300 million years ago. The journey resembles a visit back to a long-missed hometown," the prologue narrates in every episode.

Liao says: "Compared to the sky above our heads, human beings still have much less knowledge of the sea and the world beneath it."